The invention relates to a method and apparatus for biological activating cleaning of waste waters containing nitrogenous materials using lowly loaded sludge with simultaneous nitrification of organic nitrogen and ammonia to nitrates while using fluid filtration.
In the technology of water cleaning, a target achievement is higher efficiency of cleaning such that the cleaned water may be used in closed technological cycles producing no waste water. In order to achieve this goal, it is necessary to clean waste waters not only of commonly appearing organic contaminating materials but also of nitrogenous contamination.
It is known that the majority of waste waters contain varying amounts of nitrogenous materials which are present in the water mostly in the form of organic nitrogen or ammonia. It is also known that by aerobic activating cleaning at lowly loaded activation it is possible to convert organically bound nitrogen and ammonia by microbial nitrification oxidation processes to nitrates.
It is further known that the presence of nitrates in the activating mixture due to nitrification processes causes a substantial deterioration of the functioning of cleaning arrangements due to flotation of sludge in the course of separation.
With known arrangements using fluid filtration for separation of the suspension of the activated sludge from the cleaned water, this effect is particularly conspicuous. The causes of flotation of sludge are denitrification processes in the fluid filter where gaseous nitrogen is released which sticks to particles of activated sludge in the shape of bubbles so that their specific weight is reduced to such an extent that particles of activated sludge with sticking bubbles which are lighter than water float to the surface of the liquid. This kind of flotation of sludge occurs in conditions where no solved oxygen is present in the fluid filtration layer and where high temperature of the activating mixture increases the intensity of the denitrification processes.
As the majority of waste waters contain nitrogenous materials in an amount which is sufficient for denitrification processes in the fluid filter, the flotation effect shows in a greater or lesser degree with most waters which are cleaned biologically at lowly loaded activation with simultaneous denitrification. In order to achieve a rather apparent flotation, a content of 10 mg. per liter N--NO.sub.3 in the activation mixture is sufficient. This concentration is present in practically all waste waters including sewage which are cleaned in conditions of lowly loaded activation. In order to achieve an effective separation by fluid filtration it has been therefore endeavored to reduce these denitrification processes in the fluid filter in order to reduce these disturbing influences of flotation. It is possible to achieve this by two methods: first, by increasing the content of dissolved oxygen in the activating mixture, which is secured by supplying oxygen also into the fluid filtration layer in an increased amount. A second possibility is a reduction of retention of the activating mixture in the fluid filter by reduction of its volume.
As the space of the fluid filter requires, for its correct functioning, a shape with a flow area increasing in the upward direction, which is in practice the shape of a cone or prism, the content of the fluid filter is highly dependent on its height. In order to prevent creation of conditions of presence of dissolved oxygen, it is therefore necessary to maintain the surface of the fluid filter at a small height, which secures its small volume, where the stored oxygen is sufficient for maintenance of conditions with presence of dissolved oxygen.
Preventing flotation due to denitrification has, however, substantial drawbacks. The suppression of denitrification by increased supply of oxygen to the fluid filter by its increased concentration in the activating mixture substantially increases claims on power for aeration. The reduction of the time the activating mixture is retained in the fluid filter by reduction of its volume reduces the hydraulic capacity of separation. As by reduction of its volume, also its separation surface is reduced, which reduces the capacity of the whole arrangement.
In addition to these drawbacks, the maintenance of the supply of oxygen to the fluid filter brings further drawbacks, particularly in complex systems of activating cleaning of waste waters with simultaneous nitrification and denitrification. Where the technology of denitrification using a uniform sludge is applied, the increased oxygen content in the activating mixture requires an increased volume of the denitrification space due to supply of a certain unwelcomed increased amount of oxygen into the denitrification space where there is circulation of the mixture between the aerated and non-aerated zone of activation.